Thermoelectric sensing elements made of a special-design multicore cable with insulation find ever wider applications.
Substantial advantages offered by such thermoelectric sensing elements or transducers reside mainly in the fact that they make it possible to have the minimum diameter of for example, 1.00 mm in the area of location of the hot junction (to enhance the measurement accuracy), which diameter is then increased to 3.00 mm in a smooth transition over a 50 mm length (to reduce the impedance of the measurement circuit). This enables to do without specific adaptors between the cables of different diameters and makes sensing elements of this type by far the best for such applications as the active zone of nuclear reactors and other apparatus with elevated temperatures.
There is known a method of rolling small-diameter tubes (cf. SU Inventor's Certificate No. 296,603; Int. C1. B 21 b 13/18, dated Dec. 14, 1970), comprising the following steps.
The rolling of the tubes is effected by three rollers accommodated in a specific cage and having their journals bearing upon profiled supporting strips.
The latter are mounted in a holder mounted, in its turn, in the bearing assemblies of a carriage of a welded structure, provided with a mechanism for rotating it, while the cage of the rollers is connected to the drive through a bearing unit of which the axis is aligned with the axis of the tube being rolled.
The mechanism for rotating the holder is made as a driven splined shaft with pinions mounted thereon.
In the rolling operation, the carriage is reciprocated jointly with the supporting strips mounted therein. The drive reciprocating the carriage is essentially a crank mechanism, the carriage being connected with the link arm through a rod of an adjustable length.
As the carriage is reciprocated, the link arm is driven through a rocking motion about its stationary axis. The points of connection of the rod of the cage and of the carriage to the link arm are so situated that the linear speed of the cage and the amount of its displacement along the axis of rolling are one half of those of the carriage.
When the stand is driven through the working stroke, the rollers have their journals bearing upon the inclined surfaces of the supporting strips, providing for bringing the rollers simultaneously together by the value of the predetermined difference between the heights.
The groove of the roller corresponds to the selected size of the tubes to be rolled and has its own size permanent over the entire perimeter.
When the size of tubes to be rolled is changed, the rollers are replaced and the leverage of the rolling stand is readjusted.
A tube is fed in when the stand occupies the rearmost position in the rolling direction. Simultaneously, the rotation mechanism of the rolling-stand is operated to rotate the rotatable holder and the cage with the rollers, the holder being rotated by the torque transmitted by the driven splined shaft through the pinions.
However, this known method is not free from drawbacks.
The single-pass deformation amounts to but 6 to 10 percent. This is explained by the fact that tubes are rolled by the rollers having the permanent cross-section of their grooves, so that 15 passes are required to roll tubes of the 1.0 mm diameter from 3.0 mm blanks.
With a single set of the working rollers being fit for rolling tubes of one diameter only, in the abovedescribed case 15 sets of working rollers would have been required.
The manufacture of the working tooling for rolling tubes of diameters short of 3.0 mm has proved to be so technologically complicated that the method being described has been deemed impractical both for rolling tubes and making a multicore electric cable.
There is commonly known a method of making a multicore cable by the drawing technique, including the following steps.
The leading end of a multicore cable 15 to 25 meter long (depending on the further application of the cable), coiled into a coil 400 to 500 mm in diameter, is prepared for being clamped in a drawing gripper, and then the necessary length of the leading end portion is drawn successively through a series of drawing dies to a diameter of 2.6 mm. Then the processed length of the cable is annealed (to relieve the strain) in a furnace filled with argon (the shielding gas) at 800.degree. C. to 1000.degree. C. for 15 minutes. The same furnace is used for annealing simultaneously several blanks of the cable being processed.
Following the annealing, the cable is subjected to similar drawing to the diameter of 2.32 mm and to another annealing operation.
Then the cable is drawn to the diameter of 1.8 mm, annealed, and drawn once again to the diameter of 1.6 mm, whereafter the gripped end is cut off, and the cable is annealed once again.
The reduction of the multicore cable from the 3.00 mm diameter to the 1.6 mm diameter is effected in 23 passes, with the outer diameter of the cable reduced by 0.06 mm during each pass, with four intervening annealing operations.
The drawing of the cable from the 1.6 mm diameter to the diameter of 1.0 mm is conducted in a similar manner, the only difference being that the total reduction of the cable is achieved in 30 passes, with the outer diameter of the cable being reduced by 0.02 mm in each pass. Then the gripped end is cut off.
Thus, the generally used technique of making an electric cable by drawing from the 3.0 mm diameter to the 1.0 mm one involves 53 passes and 8 intervening annealing operations.
A drawback of the known method is that the respective technology of making a multicore cable with the diameter varying along its length is very labor-consuming.
Moreover, the predominant action of axial forces in the deformation area in the course of drawing creates the least favorable conditions for deforming the metal, results in significantly quicker strain hardening and tends to leave bottleneck portions and to increase the breakage rate of the metal being deformed.
For this reason the drawing technology necessitates the considerable amount of passes with a small degree of deformation and intervening annealing stages intended to relieve the strain in the metal.
Quite obviously, this technology badly affects the general productivity in the fabrication of a multicore cable, to say nothing of its necessitating an increased amount of the production plant and of its operators.
There is known a method of making a multicore cable by rotary swaging or reduction (cf. V. F. Sutchkov, V. I. Svetlov, E. E. Finkel, "Heat-Resistant Cables with Magnesia Insulation", ENERGIYA Publishers, Moscow, 1969, p. 19) including the following stages.
The shape of the blank is changed by reduction in rotary swaging machines by a working member rotated about the blank and having a tool operatively connected with a mechanical drive and a reciprocation mechanism.
The blank in the reduction zone is acted upon by external compressing forces transmitted via the strikers, which causes its deformation, with the cross-section of the blank being reduced and the metal moving axially of the blank.
The accuracy and finish attained by working articles by the rotary swaging technique is greatly dependent on the manufacturing quality of the tools--the strikers, on the rigidity, the assembling quality and the adjustment of the rotary swaging mechanism.
Assuming that the abovedescribed combination of the factors is satisfactory, the machine is capable of producing a multicore cable of the 1.0 mm diameter from a blank 3.0 mm in diameter in a single pass, with the surface complying with "9" to "10" Finish Class (standard deviation of profile roughness R.sub.z of 0.16 to 0.32) and "2" to "3" Accuracy Class (tolerance J.sub.s from 10 microns to 24 microns).
However, notwithstanding the attainable high surface finish and accuracy class, a drawback of this method is that swaging in machines with revolving working tools--the strikers--results in substantial twisting of the article being worked.
The existing designs of mechanisms of rotary swaging machines do not provide for reducing the blank simultaneously over its entire contour. Therefore, the metal being deformed is allowed to flow into gaps between the strikers, i.e. the pattern allows for deformation with expansion.
This factor curbs down the rate of feeding the metal to be deformed into the deformation zone, and, therefore, puts a limit to the throughput of the machine.
The throughput of the operation of rotary swaging could be increased by stepping up the number of individual compressions per unit of time, but this would lead to increased noise, vibration, rapid failure of the components and tools, more frequent maintenance and repairs of the machine, and even to emergency situations.